A reticle is an optically clear or reflective substrate covered on one side with an opaque or light attenuating film, in which a pattern is formed by photolithography or other similar processes. Often, it is made of quartz coated with chrome. The reticle carries an image of part of an electrical circuit, which image is used as a master for repeated projection onto a semiconductor wafer to make integrated circuits. During production of the reticle, the coated side is held uppermost and the reticle is supported on its uncoated side or is held by the beveled edges/corners during handling and processing. After the reticle has passed final inspection, a pellicle may be attached to the coated side to help protect the image area from particulate deposition and chemical contamination. The reticle then is placed into a shipping box that is sealed to protect it from contamination while in transit to the semiconductor factory where it will be used. Upon arrival at the point of use, the reticle is transferred into another box which is designed so that it can be opened automatically by the lithography tool that uses the reticle. The reticle is placed in this box with the coated side (and pellicle, if fitted) facing downwards, typically resting on support points that contact the coated underside of the reticle in the region outside the pellicle frame.
The rigid structures that laterally define the reticle containment volume in the box cannot prevent the reticle from moving on the support points, as there must be clearance space between such boundary structures and the reticle to allow for dimensional and positional tolerances of the box parts and of the reticle itself, as well as the placement accuracy of the handling mechanism that moves the reticle into and out of the box. Current implementations of reticle boxes either restrict reticle movement when the box is closed by applying a downward force on the reticle to hold it against the support points or by moving a retaining mechanism against a vertical side of the reticle to hold it against a rigid vertical structure in the box. Examples of such mechanisms can be found in U.S. Pat. No. 4,815,912 and US Patent Application Publication No. 2002/0066692.
The prior art which clamps the reticle onto the support points by applying a downward force on the reticle increases the friction at the reticle support points; and if the reticle moves sideways under such conditions, for example as a result of shock loading, there is an increased risk of damage to the coating on the reticle or of particulate generation from the supports. In some designs, the sprung structures that apply the downward force to clamp the reticle are also designed to center the reticle in the pod. In such designs, lateral reticle movement under increased friction at the supports is induced every time the pod is closed. Even if the reticle does not move sideways, the application of pressure between the reticle and the support points can cause material from the supports to adhere to the reticle and/or cause material of the reticle coating to adhere to the supports. If the box is made from molded plastic material, the clamping force is likely to have a distorting effect on the walls and door of the box. It is well-known from experience with plastic Standard Mechanical Interface (SMIF) doors in multi-reticle handling that such distortions result in dimensional instability of the assembly and cause handling errors due to displacement of the reticle from the nominal plane that is addressed by the handling robot.
In the prior art designs in which the reticle is restrained in the box laterally but with no downward pressure on the reticle other than the force of gravity, there is a possibility of vertical reticle movement if the box is tipped or the reticle experiences negative g-forces during handling. This will allow the reticle to “bounce” on the supports and can also lead to damage of the coating on the reticle or to particle generation at the support points.
Particle generation at the support points in a reticle box has been identified as a cause of image distortion during exposure in a lithography tool. Such particles can become trapped between the reticle and the support surface in the lithography tool onto which the reticle is clamped, normally by vacuum. As the clamping force is large, the reticle can be distorted which causes image distortions and printing faults known as “overlay errors” which cannot be fully corrected by adjusting the settings of the lithography tool.
The latest generations of reticle boxes are designed such that the support points do not coincide with the vacuum chucking points that are used to support the reticle in the lithography tool, and some lithography tool chucks have been designed to be insensitive to the presence of particles. Nevertheless, damage to the coating or the generation of particles from the supports on the underside of a reticle is always undesirable, since particles can be transferred to other sensitive areas of the reticle or processing equipment by subsequent handling and by air showers.
Some reticles are manufactured with the opaque coating removed in the regions of the reticle contact points so that the coating cannot be damaged and cause particle generation, for example, as described in US Patent Application Publication Nos. 2005/0229145 and 2004/0005209. However, particle generation is not eliminated completely by removing the coating in the support areas, since the support material of the box typically is softer than that of the reticle, and it can also be damaged and generate particles through the action of pressure and/or friction when it is in contact with the reticle surface.
To reduce the generation of particles on sensitive areas of a reticle, the support points used in some reticle boxes are placed at the corners/edges so that the major surfaces of the reticle are not touched at all, as described in US Patent Application Publication Nos. 2006/0126052 and 2002/0066692 and in U.S. Pat. No. 6,216,873. However, such support configurations generally are not used in semiconductor production facilities due to their incompatibility with the designs of existing lithography equipment and reticle handling systems. The use of boxes employing such support points, therefore, is restricted to reticle shipment or within reticle manufacturing areas, so reticles must be transferred from this type of box to another type at least once during the reticle's journey from its place of manufacture to its point of use in a semiconductor fabrication facility. This exposes reticles to increased risk of contamination and electrostatic damage, especially if the transfer is done manually. Automating this transfer of reticles between boxes increases cost, occupies valuable clean room space with handling equipment, and introduces extra reticle handling steps.